Load-generated drive, substantially no quiescent current, techniques and circuits for high speed switching of transistors

ABSTRACT

Techniques and circuits for high speed switching of transistors are provided. These techniques and circuits switch an output device while varying the drive current to the output device in proportion to the output current through the output device. In addition, these techniques and circuits provide a switching circuit with substantially no quiescent currents. This is accomplished by sampling the output current conducted by the output device and using the sample as a signal to drive either the output device fully ON or to switch the output device fully OFF.

This is a continuation of application Ser. No. 09/397,629, filed Sep.15, 1999 (CPA filed Feb. 6, 2001) (now U.S. Pat. No. 6,531,909). Thisprior application is hereby incorporated by reference herein in itsentirety.

BACKGROUND OF THE INVENTION

This invention relates to switching circuits. More particularly, thisinvention relates to switching circuits having independent source andsink components.

One conventional method of switching ON an output circuit, e.g., theoutput transistor of a switching regulator, is to drive its base highwith a fixed current source coupled to Vcc. After the circuit isswitched ON, an anti-saturation circuit may be used to detect thevoltage across the output circuit, and, when necessary, to shunt excessdrive to ground or into the collector of the output circuit. When theoutput circuit is switched OFF, a fixed current sink may be used to shutOFF the output circuit by sinking current from the base of the outputcircuit to ground.

This approach, however, has several potential problems. One problem isthat the source and sink currents required to drive the output circuitare often excessive under lighter loads. Under such conditions, power iswasted when the output circuit is switched ON by a source circuit to agreater extent than necessary, and when the output circuit is switchedOFF by a sink circuit to a greater extent than necessary. In addition,the extra source current supplied by the source circuit under lighterloads may slow the switch OFF time of the source circuit. Because theoutput circuit will not switch OFF as long as the source circuit isstill ON, a delay in switching OFF the source circuit lengthens theswitch-OFF period of the output circuit.

Another potential problem with conventional switching circuits is thatthe sink circuit often must be switched OFF itself before the outputcircuit can be switched back ON. This also may add extra time delay tothe switch-ON period of the output circuit.

One other problem that may occur is that additional circuits may berequired to prevent the output circuit from saturating. These circuits,which typically shunt extra drive current to ground, rely on an assumedsaturation voltage for reference. Thus, these circuits typically have nocompensation for variations in transistor characteristic or loadcurrent.

A problem may also exist if the emitter of the output circuit is heldabove ground during the switch OFF period, and the sink circuit groundsthe base of the output circuit. This arrangement can backward bias theemitter-base junction of the output circuit and cause an undesirableleakage path from the emitter of the output circuit through its base toground via the sink circuit. This leakage can damage the output circuit.

Finally, if the load is short-circuited, the source circuit may beforced to provide maximum drive current. This can cause the outputcircuit to operate at excessively high currents, potentially causingdamage to the output circuit.

SUMMARY OF THE INVENTION

Therefore, it is an object of this invention to provide a switchingcircuit that conserves power and reduces the switch-OFF time by limitingthe excess current provided by the source and sink circuits in theswitching circuit.

It is also an object of this invention to provide a switching circuitwhich reduces the switch-ON period by minimizing delays in switching OFFthe sink circuit.

It is also an object of this invention to provide a switching circuitthat compensates for variations in transistor characteristics and/orload current.

It is also an object of this invention to provide a switching circuitwhich is not subject to damage caused by back-biasing of the outputcircuit emitter-base junction.

It is also an object of this invention to provide a switching circuitwhich is not subject to damage when the load is short-circuited.

Therefore, switching circuits and techniques including output circuits,switch-ON circuits, switch-OFF circuits and controlled current sourcesare provided. A single input signal, which is either ON or OFF, is fedto both the switch-ON circuit and the switch-OFF circuit. In response toan ON signal, the switch-ON circuit provides a switch-ON current toinitiate the turn-ON of the output circuit. A sample of the outputcurrent, e.g. the collector-emitter current in an output transistoracting as the output circuit, is then sampled and fed to the controlledcurrent source which then drives the output circuit to a higher level.At this higher level, the output circuit may be switched fully ON. Inresponse to an OFF signal, the switch-OFF circuit operates to sink drivecurrent away from the output circuit to initiate turn-OFF of the outputcircuit. A sample of the output current is then fed to the controlledcurrent source which then drives the switch-OFF circuit to a higherlevel. At this higher level, the switch-OFF circuit is able to fullyswitch OFF the output circuit.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects and advantages of the invention will beapparent upon consideration of the following detailed description, takenin conjunction with the accompanying drawings, in which like referencecharacters refer to like parts throughout, and in which:

FIG. 1 is a block diagram of a switching circuit according to theprinciples of the invention.

FIG. 2 is a circuit diagram of a switching circuit according to theprinciples of the invention.

FIG. 3 is a circuit diagram of a switching circuit implementing acascode configuration according to the principles of the invention.

FIG. 4 is a circuit diagram of a switching circuit implementingDarlington configurations according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Circuits and techniques for providing high-speed, load-generated, noquiescent current switching of transistors are provided. Thesetransistors may have floating emitters.

In conventional switching circuits, a switch-ON circuit may be used todrive the base of an output device HIGH, thereby switching ON the outputdevice when the output device has a source of constant current supplyingits base-emitter junction. When the output device is ON, ananti-saturation circuit may be used to shunt excess current from thebase of the output device to ground. A switch-OFF circuit may be used tosink current from the base of the output device, thereby switching OFFthe output device.

However, conventional switching circuits present a number of potentialproblems, as described above. Many of the problems may involveregulation of the currents in the switch-ON circuit and switch-OFFcircuit. A switching circuit constructed according to the inventionovercomes these problems by sampling the current provided to the emitterof the output device, and using that sample to regulate and augment thedrive currents to the output circuit and the switch-OFF circuit.

A switching circuit according to the principles of the inventionoperates as follows. First, an input signal is provided to the circuit.The input signal can be either HIGH or LOW (or, in the alternative, ONor OFF) and is generated by any suitable method known in the art ofproducing a HIGH or LOW input. Generation of such signals is well-knownin the art and does not require further explanation. When the inputsignal is HIGH, current is preferably injected into the base of theoutput device via the switch-ON circuit. A controlled current source (inone particular embodiment, the controlled current source is a currentmirror) then senses the current in the output device, e.g. through thecollector-emitter of the output device, and preferably drives the baseof the output device even higher. This additional current then fullyturns ON the output device.

When the input signal is LOW, current is preferably drawn out of thebase of the output device by the switch-OFF device. The controlledcurrent source then senses the current in the output device, andpreferably provides additional drive to the switch-OFF circuit which, inturn, draws out even more current from the base of the output device.This additional current drain then turns OFF the output device.

In one aspect of the invention, the switching circuit provides addedefficiency because the switch-OFF device does not draw any current oncethe output device is switched OFF. Therefore, when the input signal isswitched ON, the switch-ON circuit can source current immediately intothe base of the output device because there are no delays in waiting forthe switch-OFF device to stop drawing current.

Another aspect of the invention involves rapidly starting the switch-OFFcircuit. When the input signal is switched LOW, the switch-ON circuit isswitched OFF. Because the controlled current source, e.g., the currentmirror, is already providing current at the transition of the switchingcircuit from ON to OFF (it was just maintaining the switch-ON circuit inan ON state), the switch-OFF circuit can rapidly switch ON because it isalso supplied with current from the controlled current source.Therefore, the switch-OFF circuit can start drawing current from thebase of the output device immediately at the onset of the OFF period ofthe input signal. This provides another saving in circuit efficiency.

Another aspect of the invention is that it maintains the beta of theoutput device at a pre-determined level. This level is determined by theratio of the emitter-base junction area of the transistors in thecontrolled current source to each other and to the output device. Bysetting the beta of the output device close to the point of saturation,the output device retains only a minimum amount of excess base charge.This reduces shut-OFF time because there is minimal delay attributableto removing excess base charge from the output device.

The beta of the output device is preferably set at some fixed ratio toone of the transistors in the controlled current source. This allows thecontrolled current source to sample a portion of the current conductedby the output device and use that sample to further drive either theoutput device or the switch-OFF circuit. In one embodiment, the currentin the output transistor is sampled by coupling the emitter and the baseof the output transistor to the emitter and base of the transistor inthe controlled current source, respectively. The matching between thesetransistors can be further improved by forcing the collector voltage ofthe current source transistor to approximately match the collectorvoltage of the output transistor. This can be accomplished by couplingthe respective collectors in a cascode formation, as will be explained.

Another aspect of the invention is that the circuit is arranged in sucha fashion that the quiescent current is eliminated in the OFF state.This is because the switch-OFF circuit is driven by a current which iscontrolled, in one embodiment, by a current from the output device.Therefore, when the output device is completely shut OFF, e.g., afterits base is drained, the switch-OFF circuit is also OFF. In addition, ifthe output transistor begins to leak current, the switch-OFF circuitwill be activated to hold the output device OFF.

FIG. 1 shows a block diagram of switching circuit 10 according to theinvention. First, operation of this circuit will be described tointroduce the concepts of the invention. Then, the invention will bedescribed in more detail with reference to FIGS. 2, 3 and 4.

In FIG. 1, an input signal is fed into both a switch-ON circuit 20 and aswitch-OFF circuit 30. The input signal is either HIGH or LOW. When theinput signal is HIGH, it turns the switch-ON circuit 20 ON. Theswitch-ON circuit 20 then provides a current to the output device 40.This current, however, is not sufficient to fully drive output device10.

At this point, a controlled current source 50 is switched ON because itresponds to any current conducted by output device 40. In oneembodiment, controlled current source 50 then begins driving additionalcurrent into output device 40 directly. This additional current allowsoutput device 40 to be fully turned ON.

When input signal switches OFF, it turns OFF switch-ON circuit 20 andturns ON switch-OFF circuit 30. Switch-OFF circuit 30 promptly beginsdraining current from the base of output device 40 because controlledcurrent source 50 is already producing sufficient current to driveswitch-OFF circuit 30 at a high level. When the base of output device 40is drained, no current is mirrored to switch-OFF circuit 30, and itturns OFF. Thus, in the OFF state, all transistors in switching circuit10 are OFF, and switching circuit 10 draws substantially no quiescentcurrent.

FIG. 2 is a circuit diagram of one embodiment of a switching circuit 100according to the invention. This circuit is more detailed than the blockdiagram in FIG. 1, but the principles are essentially the same.

Transistor 110 is a large output device having an emitter that may be,for example, 100 times greater the area than the emitter of transistor120. It follows that transistor 120 operates at {fraction (1/100)}th theoutput current conducted through the collector-emitter junction ofoutput transistor 110.

Transistor 120 is part of controlled current source also comprisingtransistors 120, 130, 140 and 170 which mirrors a current throughtransistor 120 for use by the rest of the circuit. When the input signalis ON, transistor 140 provides base drive to turn output transistor 110fully ON. When the input signal is OFF, transistor 170 provides basedrive to transistor 150, which, when activated, sinks current from thebase of output transistor 110.

Switching circuit 100 operates as follows: when the input signal isturned HIGH, current source 105 is signaled to inject current intooutput transistor 110's base. This current accomplishes two tasks:first, it slightly turns ON output transistor 110, and second, itinitiates operation of controlled current source transistors 120, 130,140 and 170. Thus, as output transistor 110 begins to turn ON, thecontrolled current source, comprising transistors 120, 130, 140 and 170,turns on as well.

Additional current provided by transistor 140 then provides base currentto output transistor 110 until it is fully ON. Thus, the switch-ONcircuit, as mentioned in reference to FIG. 1, may include, for example,as shown in FIG. 2, current source 105. The switch-OFF circuitpreferably may include, for example, transistors 150 and 160, and diode115. The controlled current source may include, for example, transistors120, 130, 140 and 170, and resistor 125.

When the input signal is HIGH, transistor 160 holds transistor 150'sbase OFF because transistor 160 drains any current that may be providedby transistor 170. Resistor 125 limits the maximum current transistor160 has to divert from the base of transistor 150. Schottky diode 115may be used to prevent transistor 160 from drawing current from theemitter-base junction of transistor 150 when transistor 150 has no basecurrent.

In the preferred embodiment of switching circuit 100, the ratio oftransistor 110 to transistor 120, as mentioned above, is 100:1, and theratio of transistor 130 to transistor 140 is 1:2. (These ratios, as wellas all other ratios and values specified in this patent, are onlyexemplary embodiments of the invention, and are not intended to limitthe scope of the invention to the particular values.) Therefore, outputtransistor 110 has a forced beta of 50 because the current that isprovided to its base will always be approximately {fraction (1/50)} ofthe current it conducts through its collector-emitter junction. Thisbeta remains constant even when transistor 110 nears saturation,irrespective of load or saturation voltage.

When the input signal is turned OFF, current source 105 and transistor160 are turned OFF. Because transistor 170 is already sourcing current,transistor 150 switches ON rapidly.

To understand how the invention provides a shortened turn-OFF time, theturn-OFF process for conventional switching circuits should beconsidered. Typically, the turn-OFF time of an output transistor dependson its prior operating conditions.

As the base drive to a transistor is increased for a given load, thecollector-emitter voltage drops. In conventional circuits, this forcesthe output transistor to have a progressively lower beta. At some betapoint, an increase in base drive produces an insignificant reduction incollector-emitter voltage. The output transistor is then saturated.Thereafter, the collector-base capacitance increases rapidly, storingexcess charge on the base. This excess charge must be pulled back out ofthe base before the device will switch OFF, delaying the turn-OFFprocess of the switching circuit.

The invention overcomes this time delay problem because the forced betaof output transistor 110 operates to keep output transistor 110 just atthe point of saturation. Therefore, output transistor 110 has only aminimum amount of excess base charge to remove at turn-OFF. Thissignificantly reduces the turn-OFF time of output transistor 110.

Another advantage of the preferred embodiment of the invention is thatbecause transistor 150 is driven by a controlled current source which iscoupled to the base of output transistor 110, transistor 150 will onlybe driven as long as there is current in output transistor 110. Thus,when output transistor 110 is completely turned OFF, all the othertransistors, including the transistors in the switch-OFF circuit, willalso be OFF. It follows that this embodiment of the circuit has noquiescent current. This also allows current source 105 to source currentimmediately to the base of output transistor 110 at turn-ON becausethere are no delays in waiting for transistor 150 to turn OFF.

Another advantage of this configuration is that if output transistor 110leaks so as to turn itself back ON, transistor 120 will mirror the leakand activate transistor 150 to hold transistor 110 OFF. Unless disabled,the turn-OFF loop will always dominate the turn-ON loop due because ofthe extra gain from transistor 150.

Additional advantages of the invention are as follows. The circuit canoperate with the output device at any voltage between supply and groundbecause the drive current for the switch-ON circuitry and the switch-OFFcircuitry are proportional to the output current, and not to anindependent current source. The circuit also requires no saturationreference device or voltage because the beta of the output transistor isfixed at some pre-determined level (the fixed beta also protects theoutput transistor under high load currents because it does not allow avery high current to be passed through the output transistor but,rather, limits the current in the output transistor to somepre-determined ratio to the base current). Therefore, the circuit doesnot require anti-saturation circuitry to detect excess drive current atthe base of the output transistor and shunt the excess drive current toground. Additionally, the drive circuit adapts to changing load currentand therefore only a relatively small amount of current is lost toground.

FIG. 3 shows a switching circuit 200 having a cascode configuration atthe connection between the output device and the controlled currentsource. Reference transistor 210, which is supplied by current source220, and cascode transistor 230 are configured such that the respectivecollectors of transistors 110 and 120 are each one V_(be) lower than thebase of transistor 210. Thus, in this configuration, the base, emitterand collector of transistor 110 are matched to the base, emitter andcollector of transistor 120, respectively. This architecture improvesthe accuracy of the controlled current source's sampling of the currentin the output circuit and, thereby, improves efficiency of the circuit.

FIG. 4 shows a switching circuit 300 that expands the approach of theinvention to higher current devices requiring Darlington drive stages.In FIG. 4, transistor 320 provides the Darlington drive to the base ofoutput transistor 110, effectively increasing its base current by theemitter ratio of transistor 310 to transistor 320.

In switching circuit 300, the ratio of output transistor 110 totransistor 120 is preferably about 500:1. The ratio of transistor 130 totransistor 140 is preferably 1:1. The ratio of transistor 320 totransistor 310 is preferably 1:10 (transistors 320 and 310 add into thebase of transistor 110). These ratios, as in the circuit shown in FIG.2, force output transistor 110 to have a beta of about 50.

A Darlington structure can also be used in the switch-OFF circuit, asshown in FIG. 4. At turn-OFF, transistor 330 removes current that wasdriving transistor 320's base and uses it together with transistor 160'scurrent, as provided by is transistor 170, to drive transistor 150.After turn-OFF, no quiescent current is drawn.

Thus it is seen that switching circuits and techniques having loadgenerated drive and substantially no quiescent current for high speedswitching of transistors have been provided. Persons skilled in the artwill appreciate that the present invention can be practiced by otherthan the described embodiments, which are presented for purposes ofillustration rather than of limitation, and the present invention islimited only by the claims which follow.

1. A switching circuit that switches an output circuit, said switchingcircuit comprising: a switch-ON circuit that provides a switch-ONcurrent to said output circuit when an input signal is ON; a switch-OFFcircuit that operates when said input signal is OFF; and a controlledcurrent source coupled to said switch-ON circuit, said switch-OFFcircuit, and said output circuit, wherein said controlled current sourcecontinually samples, using a current mirror, an output current from saidoutput circuit and provides drive current to said output circuit inproportion to said output current, and provides at least a portion ofsaid drive current to said switch-OFF circuit if said input signal isOFF.
 2. The switching circuit of claim 1, wherein said switch-OFFcircuit operates to remove said drive current from said output circuit.3. The switching circuit of claim 1, wherein said output circuitcomprises a transistor.
 4. The switching circuit of claim 3, wherein abase of said transistor is coupled to be driven by said switch-ONcurrent.
 5. The switching circuit of claim 3, wherein a base of saidtransistor is coupled to be driven by said drive current.
 6. Theswitching circuit of claim 1, wherein said switch-ON circuit comprises acurrent source coupled to receive said input signal.
 7. The switchingcircuit of claim 1, said controlled current source comprising a firsttransistor coupled to sample said output current.
 8. The switchingcircuit of claim 7, wherein said output circuit is an output transistorand the ratio of said output transistor to said first transistor isfixed.
 9. The switching circuit of claim 1, wherein said controlledcurrent source circuit is configured to supply substantially no currentto said switch-ON circuit and substantially no current to saidswitch-OFF circuit when said output current is substantially zero.
 10. Amethod for using a switching circuit to switch an output circuit, saidswitching circuit comprising a switch-ON circuit, a switch-OFF circuitand a controlled current source, said method comprising: providing aninput signal to said switch-ON circuit and to said switch-OFF circuit;when said input signal is ON, said switch-ON circuit providing aswitch-ON current to said output circuit; when said input signal is OFF,operating said switch-OFF circuit; using said controlled current sourceto continually sample, using a current mirror, an output currentconducted by said output circuit; providing a drive current from saidcontrolled current source to said output circuit when said input signalis ON and providing at least a portion of said drive current to saidswitch-OFF circuit when said input signal is OFF, said drive currentbeing proportional to said output current.
 11. The method of claim 10wherein said providing a switch-ON current comprises providing saidswitch-ON current to a base of a transistor in said output circuit. 12.The method of claim 10 wherein said providing a drive current comprisesamplifying a sample of said output current to provide said drivecurrent.
 13. The method of claim 10 further comprising preventing anundesired turn-ON of said switch-OFF circuit.
 14. The method of claim 10wherein said providing drive current comprises providing a substantiallyzero current when said input signal and said output current aresubstantially zero.
 15. A switching circuit to switch an output circuit,said switching circuit comprising a switch-ON circuit, a switch-OFFcircuit and a controlled current source, said switching circuit furthercomprising: a means for providing an input signal to said switch-ONcircuit and to said switch-OFF circuit; when said input signal is ON, ameans for providing a switch-ON current from said switch-ON circuit tosaid output circuit; when said input signal is OFF, a means foroperating said switch-OFF circuit; a means for using said controlledcurrent source to continually sample, using a current mirror, an outputcurrent conducted by said output circuit; a means for providing a drivecurrent from said controlled current source to said output circuit whensaid input signal is ON and providing at least a portion of said drivecurrent to said switch-OFF circuit when said input signal is OFF, saiddrive current being proportional to said output current.
 16. Theswitching circuit of claim 15 wherein said means for providing aswitch-ON current is comprised to a base of a transistor in said outputcircuit.
 17. The switching circuit of claim 15 wherein said means forproviding a drive current comprises a means for amplifying a sample ofsaid output current to provide said drive current.
 18. The switchingcircuit of claim 15 wherein said means for operating said switch-OFFcircuit comprises a means for preventing the undesired turn-ON of saidswitch-OFF circuit.
 19. The switching circuit of claim 15 wherein saidmeans for providing a drive current is configured to allow said drivecurrent to be conducted at a fixed ratio to said output current.
 20. Theswitching circuit of claim 15 wherein said means for providing a drivecurrent comprises a means for providing substantially zero current wheninput signal is OFF and said switch-OFF circuit is conductingsubstantially no current.
 21. A switching circuit that switches anoutput circuit, said switching circuit comprising: a switch-ON circuitthat provides a switch-ON current to said output circuit when an inputsignal is ON; a switch-OFF circuit that operates when said input signalis OFF; and a controlled current source comprising a current mirror, thecontrolled current source coupled to said switch-ON circuit, saidswitch-OFF circuit, and said output circuit, wherein, when said inputsignal is ON, said controlled current source continually provides drivecurrent to said output circuit in proportion to said output current, theproportionality being derived using the current mirror and provides atleast a portion of said drive current to said switch-OFF circuit if saidinput signal is OFF.
 22. The switching circuit of claim 21, wherein saidswitch-OFF circuit operates to remove said drive current from saidoutput circuit.
 23. The switching circuit of claim 21, wherein saidoutput circuit comprises a transistor.
 24. The switching circuit ofclaim 23, wherein a base of said transistor is coupled to be driven bysaid switch-ON current.
 25. The switching circuit of claim 23, wherein abase of said transistor is coupled to be driven by said drive current.26. The switching circuit of claim 21, wherein said switch-ON circuitcomprises a current source coupled to receive said input signal.
 27. Theswitching circuit of claim 21, wherein said controlled current sourcecircuit is configured to supply substantially no current to saidswitch-ON circuit and substantially no current to said switch-OFFcircuit when said output current is substantially zero.
 28. A method forusing a switching circuit to switch an output circuit, said switchingcircuit comprising a switch-ON circuit, a switch-OFF circuit and acontrolled current source, said method comprising: providing an inputsignal to said switch-ON circuit and to said switch-OFF circuit; whensaid input signal is ON, said switch-ON circuit providing a switch-ONcurrent to said output circuit; when said input signal is OFF, operatingsaid switch-OFF circuit; continually providing a drive current from saidcontrolled current source to said output circuit when said input signalis ON and providing at least a portion of said drive current to saidswitch-OFF circuit when said input signal is OFF, said drive currentbeing proportional to said output current, the proportionality beingderived using a current mirror.
 29. The method of claim 28 wherein saidproviding a switch-ON current comprises providing said switch-ON currentto a base of a transistor in said output circuit.
 30. The method ofclaim 28 wherein said providing a drive current comprises amplifying asample of said output current to provide said drive current.
 31. Themethod of claim 28 further comprising preventing an undesired turn-ON ofsaid switch-OFF circuit.
 32. The method of claim 28 wherein saidproviding a drive current comprises providing substantially zero currentwhen said input signal and said output current are substantially zero.33. A switching circuit to switch an output circuit, said switchingcircuit comprising a switch-ON circuit, a switch-OFF circuit and acontrolled current source, said switching circuit comprising: a meansfor providing an input signal to said switch-ON circuit and to saidswitch-OFF circuit; when said input signal is ON, a means for providinga switch-ON current from said switch-ON circuit to said output circuit;when said input signal is OFF, a means for operating said switch-OFFcircuit; a means for continually providing a drive current from saidcontrolled current source to said output circuit when said input signalis ON and providing at least a portion of said drive current to saidswitch-OFF circuit when said input signal is OFF, said drive currentbeing proportional to said output current, the proportionality beingderived using a current mirror.
 34. The switching circuit of claim 33wherein said means for providing a switch-ON current is coupled to abase of a transistor in said output circuit.
 35. The switching circuitof claim 33 wherein said means for providing a drive current comprises ameans for amplifying a sample of said output current to provide saiddrive current.
 36. The switching circuit of claim 33 wherein said meansfor operating said switch-OFF circuit comprises a means for preventingthe undesired turn-ON of said switch-OFF circuit.
 37. The switchingcircuit of claim 33 wherein said means for providing a drive current isconfigured to allow said drive current to be conducted at a fixed ratioto said output current.
 38. The switching circuit of claim 33 whereinsaid means for providing a current comprises a means for providingsubstantially zero current when input signal is OFF and said switch-OFFcircuit is conducting substantially no current.
 39. A switching circuitthat switches an output circuit, said switching circuit comprising: aswitch-ON circuit that provides a switch-ON current to said outputcircuit when an input signal is ON; a switch-OFF circuit that operateswhen said input signal is OFF; and a controlled current source coupledto said switch-ON circuit, said switch-OFF circuit, and said outputcircuit, wherein said controlled current source continually mirrors anoutput current, using a current mirror, from said output circuit andprovides drive current to said output circuit in proportion to saidoutput current, and provides at least a portion of said drive current tosaid switch-OFF circuit if said input signal is OFF.
 40. The switchingcircuit of claim 39, wherein said switch-OFF circuit operates to removesaid drive current from said output circuit.
 41. The switching circuitof claim 39, wherein said output circuit comprises a transistor.
 42. Theswitching circuit of claim 41, wherein a base of said transistor iscoupled to be driven by said switch-ON current.
 43. The switchingcircuit of claim 41, wherein a base of said transistor is coupled to bedriven by said drive current.
 44. The switching circuit of claim 39,wherein said switch-ON circuit comprises a current source coupled toreceive said input signal.
 45. The switching circuit of claim 39, saidcontrolled current source comprising a first transistor coupled tomirror said output current.
 46. The switching circuit of claim 45,wherein said output circuit is an output transistor and the ratio ofsaid output transistor to said first transistor is fixed.
 47. Theswitching circuit of claim 39, wherein said controlled current sourcecircuit is configured to supply substantially no current to saidswitch-ON circuit and substantially no current to said switch-OFFcircuit when said output current is substantially zero.
 48. A method forusing a switching circuit to switch an output circuit, said switchingcircuit comprising a switch-ON circuit, a switch-OFF circuit and acontrolled current source, said method comprising: providing an inputsignal to said switch-ON circuit and to said switch-OFF circuit; whensaid input signal is ON, said switch-ON circuit providing a switch-ONcurrent to said output circuit; when said input signal is OFF, operatingsaid switch-OFF circuit; using said controlled current source tocontinually mirror an output current, using a current mirror, conductedby said output circuit; providing a drive current from said controlledcurrent source to said output circuit when said input signal is ON andproviding at least a portion of said drive current to said switch-OFFcircuit when said input signal is OFF, said drive current beingproportional to said output current.
 49. The method of claim 48 whereinsaid providing a switch-ON current comprises providing said switch-ONcurrent to a base of a transistor in said output circuit.
 50. The methodof claim 48 wherein said providing a drive current comprises amplifyinga mirrored current of said output current to provide said drive current.51. The method of claim 48 further comprising preventing an undesiredturn-ON of said switch-OFF circuit.
 52. The method of claim 48 whereinsaid providing a drive current comprises providing substantially zerocurrent when said input signal and said output current are substantiallyzero.
 53. A switching circuit to switch an output circuit, saidswitching circuit comprising a switch-ON circuit, a switch-OFF circuitand a controlled current source, said switching circuit comprising: ameans for providing an input signal to said switch-ON circuit and tosaid switch-OFF circuit; when said input signal is ON, a means forproviding a switch-ON current from said switch-ON circuit to said outputcircuit; when said input signal is OFF, a means for operating saidswitch-OFF circuit; a means for using said controlled current source tocontinually mirror, using a current mirror, an output current conductedby said output circuit; a means for providing a drive current from saidcontrolled current source to said output circuit when said input signalis ON and providing at least a portion of said drive current to saidswitch-OFF circuit when said input signal is OFF, said drive currentbeing proportional to said output current.
 54. The switching circuit ofclaim 53 wherein said means for providing a switch-ON current is coupledto a base of a transistor in said output circuit.
 55. The switchingcircuit of claim 53 wherein said means for providing a drive currentcomprises a means for amplifying a mirrored current of said outputcurrent to provide said drive current.
 56. The switching circuit ofclaim 53 wherein said means for operating said switch-OFF circuitcomprises a means for preventing the undesired turn-ON of saidswitch-OFF circuit.
 57. The switching circuit of claim 53 wherein saidmeans for providing a drive current is configured to allow said drivecurrent to be conducted at a fixed ratio to said output current.
 58. Theswitching circuit of claim 53 wherein said means for providing a drivecurrent comprises a means for providing substantially zero current wheninput signal is OFF and said switch-Off circuit is conductingsubstantially no current.